An approximate theory of nematic disorder, reminiscent of the spin wave theory of ferromagnetic disorder, was put forward in previous papers in this series. Predictions based upon it are here compared with accurate results for two lattice models: (a) a model in which the interaction which causes molecules to align is between nearest neighbours only, which Zannoni (1979) has investigated by computer simulation, and (b) a model in which the interaction is of very long range, to which the mean field theory ofMaier & Saupe (1958,1959, i960) is applicable. At temperatures not too close to the nematic-isotropic transition temperature Tc the theory predicts the magnitude of S2 with remarkable precision for both models, though no adjustable parameters are involved. It also predicts correctly the magnitude and the range of the short-range order that is characteristic of model (a). Since some of the discrepancies that become apparent as Tc is approached can be attributed to an approximation of rather secondary importance - neglect of the effect that misalignment entropy may have on the Frank stiffness constants of a nematic - it looks as though the theory can safely be used to describe real nematics at temperatures such that S 2 exceeds, say, 0.5. A prediction that S4(= (P4(cos 0))) is equal to however, which is certainly correct for both models at very low temperatures, begins to fail, though not dramatically, when S2 reaches about 0.7. This failure is attributed to errors in the random phase approximation, to which the other results of the theory are relatively insensitive.
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T. E. Faber (1980) studied this question.